In general, piezoelectric materials are ABO3 perovskite-type metal oxides such as lead zirconate titanate (hereinafter referred to as “PZT”). However, since PZT contains lead as an A site element, its influence on environment is controversial. Accordingly, there is a demand for a piezoelectric element of a lead-free perovskite-type metal oxide.
Barium titanate is known as a piezoelectric material of a lead-free perovskite-type metal oxide. In addition, in order to improve the characteristics of a piezoelectric material, a material of which base composition is barium titanate has been developed. PTL 1 and NPL 1 disclose materials having improved piezoelectric properties by replacing a part of the A site of barium titanate with Ca and a part of the B site with Zr. These materials, however, have low Curie temperatures, such as 80° C. or less, and thereby cause depolarization under a high-temperature environment, e.g., in a car in summer, to reduce the piezoelectric properties. In addition, since their mechanical quality factors are low, depolarization tends to occur when an AC voltage is applied to.
PTL 2 and NPL 2 disclose materials in which a part of the A site of barium titanate is replaced with Ca and to which Mn, Fe, or Cu is added. Though these materials have excellent mechanical quality factors compared to barium titanate, unfortunately, their piezoelectric properties are low.